When water is poured from a round container into a square one, the water will not retain its round shape. The water is a liquid. The liquid takes the shape of the square container. The liquid will not expand to fill a larger space. It cannot be made smaller by squeezing or pulling. The molecules do not move closer or further apart.

Gases take the shape of their containers, and expand or contract to fill the container. When a person takes a breath of air, air rushes down the bronchial tubes and tries to fill all the spaces in the lungs. A big breath makes it easier to feel the lungs expand, but a small breath fills ALL of the lungs, too. A gas can be expanded or compressed. The molecules can move closer or farther apart.

In science and engineering the word fluid refers to a liquid or a gas.

Although matter exists in these three states: solid, liquid, and gas, sometimes a substance does not seem to fit in any of these categories. A substance may behave or have characteristics of more than one of these states. Some materials like asphalt and lead initially act like a solid, but over long periods of time they begin to act like a fluid.

Is toothpaste a solid or a liquid? Actually, it has characteristics of both. If you leave the toothpaste tube cap off, the toothpaste remains inside the tube. The toothpaste does not flow out of the tube like a liquid. The toothpaste retains its shape and acts like a solid. When you squeeze the toothpaste tube, toothpaste flows out of the tube like a liquid. A certain amount of force must be applied to the toothpaste before it will move. Toothpaste acts like a solid and a liquid.

The words big, tall and gray can all describe the same object at the same time -- the words big, tall and gray can describe both a building or a large rock formation. The words colloid and nonnewtonian describe different characteristics of substances like mayonnaise and the funny putty you are making in this experiment.

Colloids - Fog, Whipped Cream, Mayonnaise and Smoke

Colloids are mixtures of a substance suspended in another substance. The suspended materials are so tiny, only 1 to 100 nanometers (10^-9 meters) long, that they do not sink to the bottom of the other substance. Materials like colloids resist small stresses but yield (move) with large stresses and begin to flow like fluids. Some examples of colloids are: smoke (solid particles suspended in air), fog, mayonnaise, meringue and the funny putty that you will make in this lesson.

In 1678, Sir Isaac Newton formulated a law which describes the movement of fluids when a shearing force is applied. (A shearing force occurs when you slide one thing over another.) Today, scientists refer to liquids and gases which move according to that law as newtonian fluids. For newtonian fluids the change in velocity is proportional to the amount of shear stress. Common liquids and gases which are newtonian include: water, oil, mercury, gasoline, alcohol, air, helium, hydrogen, and steam.

Fluids which do not follow that law are called nonnewtonian. An example of a nonnewtonian fluid is toothpaste, which does not flow out of the tube until a certain amount of force is applied by squeezing - not just any little force. You know from experience that you can squeeze a toothpaste tube a little bit and the toothpaste will not come out of the tube.

The funny putty you make in this activity is an example of a nonnewtonian colloid. It takes a firm pull to make the funny putty stretch. What happens if you pull the funny putty very quickly? Does it break in two?